My Dear Son:
Another aspect of, or way of understanding our ability to judge, is that we are unable to make an evaluation of something we cannot understand, identify, or with which we are not familiar. So the idea is, you are not able to observe that someone is cheating on his test unless you understand the concept of cheating yourself.
In the above example, you may have cheated on an exam before yourself, and that is how you understand it. Or, if you've never had that experience yourself, you nonetheless have developed a framework for understanding it, or you could not identify it, name it, or call it as it is.
Although, say, you may never have cheated on an exam, you may have had the experience of getting something that you felt you didn't deserve, or you felt was a little unfair to someone else. Our minds can make associations with experiences we have had or felt to decipher ones we haven't.
And, there are those times where we walk into an event just frankly not seeing it coming. Those are sometimes painful events that, as a mother, I would wish to spare you from. But life inevitably offers its blind spots. More thoughts on blind spots coming up.